Control valves for fluid lines, especially so-called ball valves, generally comprise a valve housing having an inlet and an outlet and between the inlet and the outlet a rotary-valve member which is generally of spherical configuration and has a passage alignable with the inlet passage and with the outlet passage in the "open" position of the valve or cock but rotatable about an axis perpendicular to the axis of the passage in this valve body, to block flow through the valve.
Such valves can be used to cut off flow along a fluid path and have the advantage that they can be actuated rapidly, are highly compact and can ensure blockage of flow and, even more importantly, flow of the fluid without significant disruption of the flow pattern in the sense that turbulence formation is minimized and thus the noise resulting from the presence of the valve or its operation is minimized.
There are two basic types of ball valves. A first of these types is generally termed a floating ball valve and is intended generally to be used in installations working at relatively low pressure and/or with units which have comparatively small dimensions.
In such a valve, during rotation of the ball into its closed position, the ball may undergo a slight translatory movement in the direction of flow of the fluid to ensure a firm seating of the valve member against a downstream seat sealing the valve against further flow past the location at which the seat engages the valve member or ball.
In the second type of ball valve, the upstream seat is movable and can be biased by fluid pressure against the valve member or ball. These valves are used for valves operating at significantly higher pressures and may have greater dimensions and can be used in larger equipment than the first type of the valve described.
In the second type of valve, the valve member or ball is pivotal about a fixed axis corresponding to the axis of an actuating shaft and lever. The biasing of the upstream seat against the valve member is ensured by a differential piston action based upon the fact that the effective cross section of the upstream seat is greater than the effective downstream cross section at which this seat engages the valve member. The seal between the seat and the valve member is provided by an elastomeric seal or the like.
When the seals engaging the ball or the surface of the latter have good coefficients of sliding friction, the opening of the valve does not present a problem even through the pressure of the fluid generates the force with which the seal engages the ball or valve member. Such valves, however, have only limited applicability since they cannot be effectively used for high temperature applications because of the detrimental effects of high temperatures on the seals which have been used. In practice, the seals which have been commonly employed have maximum operating temperatures of 150.degree. C. to 200.degree. C.